The electrowinning of aluminium from alumina dissolved in a molten salt, especially cryolite, involves unique and particularly severe materials problems linked with the corrosive conditions in the high temperature electrolyte and the reactivity of the anodically and cathodically released products.
Aluminium is still produced today by a process developed more than 100 years ago, which is called the Hall-Heroult process. In this process, both the cathode and the anode are made of carbonaceous materials.
No basic change has been made in the process despite considerable efforts by the aluminium industry and researchers to improve the process and cell design particularly by finding a substitute for carbon. All attempts to date to produce commercially-acceptable substitutes for carbon have failed: anodes are still being made of carbon in the form of pre-baked blocks or continuous Soderberg electrodes. The carbon cathodes form the bottom of the cell trough and are covered with a thick layer of aluminium which protects them from attack by cryolite and air. Even the walls of the cell are usually made of carbon and are protected by a crust of frozen cryolite. Recently, it has been proposed to replace part of the cell bottom and cell walls by other materials such as tabular alumina, see for example EP-A-0 308 013.
To find an acceptable substitute for the carbon anode seems to be an impossible task because the only materials that can resist the attack of oxygen at the temperature of electrolysis (almost 1000.degree. C.) are oxides or oxycompounds, and all oxides are more or less soluble in cryolite which was chosen particularly because aluminium oxide is soluble in cryolite.
Certain conductive ceramics can be utilized as anode structures or as anode substrates protecting anode metallic structures by having on their surface a self-sustained cerium oxide or oxyfluoride deposit which may be formed and maintained on the surface of an oxygen-evolving anode, thereby protecting the anode structure or substrate from attack by cryolite. See for example European Patents EP-B-0,114,085, EP-B-0,203,834 and U.S. Pat. Nos. 4,680,094 and 4,966,074.
Other non-consumable or only slowly consumable non-carbon anodes are for example described in European Patent EP-B-0,030,834 and U.S. Pat. No. 4,397,729.
However, the utilization of these non-carbon, non-consumable anodes has been delayed by the difficulties of retrofitting existing cells and of designing new cells in which they could be used.
To find a substitute for the carbon cathodes has been just as difficult because the only material so far found to be acceptable is titanium diboride which must be very pure and is too costly. Also, the proposed cell designs using this material have not been proven in practice.
The design of existing cells has thus remained somewhat primitive and technically inefficient and no basic improvements in the process have been made in the last 100 years, mainly due to the limitations imposed by the carbon anodes having a short life and large dimensions. However, cells of higher capacity with lower power consumption and better gas collection have been built.
Pollution remains a major problem in the operation of even the most modern cells: the pollution extends from the fabrication of pre-baked carbon blocks or from the operation of Soderberg electrodes, right through to the disposal of used carbon cathodes which are impregnated with difficult-to-dispose-of materials including cyanides.
In the conventional cells, there is non-uniform current distribution which creates irregular strong magnetic fields that produce various unwanted effects, including surface wave displacements of the thick aluminium pool at the bottom of the cell. For this reason, the anode, which is frequently replaced, cannot be held near to the cathode, resulting in a high voltage drop through the electrolyte which corresponds to approximately two-thirds of the total ohmic drop in a cell.